Ladder diagram, a special type of wiring diagram, was developed for relay-and-switching control circuits. It is named as because it resembles a ladder.
Rail and rung-Construction of ladder diagram:
The ladder diagram consists of two power rails, which are placed vertically on each side of the diagram, and rungs, which are placed horizontally between the power rails.The power rails are the source of power in the circuit (AC or DC),where the left rail is the “hot” side (voltage) and the right rail is neutral (AC) or ground (DC).
Rung in Electrical ladder diagram:
The middle rung has two switches in parallel, so only one of these switches must be closed to make the rung active. The load in this rung is a relay coil (RELAY A).The bottom rung contains a set of NO contacts from RELAY A, and the load is a motor. Consequently, when either SW 3 or SW 4 of the middle rung closes,RELAY A coil is energized, and the motor (in the bottom rung) starts.